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The addition of different concentrations (2-10 wt.%) of molybdenum disulfide (MoS2) to a poly-ether-ether-ketone matrix has been studied in terms of the thermal, mechanical and tribological properties of the materials. The results of dry-sliding tribological tests, differential scanning calorimetry and scanning electron microscope-energy-dispersive X-ray (EDS) analyses show that the concentration of MoS2 influences the tribological, mechanical and thermal properties. With the highest concentration of MoS2 (10 wt.%), the coefficient of friction was reduced by as much as 25%, while the maximum reduction in the wear rate was ~20%, which required 5 wt.% of MoS2. The most important parameter when it comes to achieving an improved tribological behaviour was found to be the combination of a high hardness and a sufficient quantity of transfer film being formed. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Ionic liquids are getting increased attention due to their potential use as additives or lubricants. One of parameters that influence their performance is anion alkyl chain length. Three chain lengths in imidazolium sulfate ionic liquids were varied in three different types of tests in order to study the friction performance in all lubrication regimes at different shear rates. Results show significant effect of alkyl chain length on friction behaviour and surface topography. The shortest chain length ionic liquid had the poorest friction performance whilst longer alkyl chain lengths performed significantly better, which was evidenced by the electrical contact resistance. Although friction behaviour was not generally reduced with ionic liquid mixtures, clear tribological improvement was observed in high shear conditions depending on alkyl chain length. Atomic force microscopy (AFM) analysis revealed correlation between ionic liquid chemistry and surface topography of samples after the experiments, with significantly increased roughness for short chain ionic liquid mixture. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In fretting wear, the microstructure and the mechanical properties of the surface and subsurface layer depend significantly on the temperature field produced in the fretting zone. Information reported in the literature indicate contradictory values for the temperature produced at the interface under fretting conditions. In the present study, a successive grinding technique was used to examine the microstructural changes generated by fretting of AISI 52100 steel and the results were compared to the mathematically calculated contact temperatures using different models. Results of this investigation indicate that white layer starts to form under individual contact spots as a result of high flash temperatures and then over the test time it grows by coalescence to a single large area.

During the forming of aluminium alloys there are several difficulties associated with controlling the process parameters and the product quality due to the severity of the contact. Adhesion and, in particular, the formation of transfer films of aluminium alloys on the surfaces of hot-work tool steels are very negative phenomena in this process and require a better understanding. The present work is focused on the initiation and evolution of the transfer of the aluminium alloy EN AW-6060 to the nitrided hot-work tool steel AISI H13 in the temperature range from 20 °C to 500 °C. All the tests were performed under cross cylinder single-pass, dry-sliding-contact conditions over different sliding distances. The contact was investigated in terms of the surface area and the volume of the transferred aluminium alloy to the surface of the tool steel, the topography of the wear trace and the corresponding coefficient of friction. The results show the strong dependence of the tribological properties in the contacts of the nitrided hot-work tool steel and the aluminium alloy on the temperature. However, it was found that the whole tribological behaviour is largely already defined during the very early stages of sliding, i.e., at the initiation of sliding, after which it becomes even more pronounced as the temperature increases. At low temperatures (20–200 °C) the surface roughness of the tool steel was found to be the key initial cause of the aluminium alloy׳s transfer, while at higher temperatures (300–500 °C) this transfer occurred predominantly due to strong adhesion. [ABSTRACT FROM AUTHOR]

Solid-lubricant nanoparticles as additives in oil provide good tribological properties based on the physical lubrication mechanisms in the contact. For this reason, they are strong candidates for use in the lubrication of diamond-like carbon (DLC) coatings, which only poorly interact with the traditional, chemically based additives. In this study, we focused on how a tribofilm formed from MoS nanotubes is related to the tribological properties of these nanotubes, and then, we analysed such a tribofilm on steel and DLC-coated surfaces using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. We demonstrated that when using oil containing MoS nanoparticles, the formation of a tribofilm is a key factor in decreasing the friction for the steel and DLC-coated contacts. The major difference between the steel and the DLC contacts is the extent to which the MoS-based tribofilm covers the surface, which is 20 % in the case of the DLC/DLC contacts, but almost 40 % in the case of the steel/steel contacts. Moreover, the MoS-based tribofilm was found to be more oxidized on the DLC surface than on the steel surface. Nevertheless, we found that the chemical and functional properties of the MoS-based tribofilm are very similar, or even the same, for both the steel and DLC-coated surfaces. No direct evidence of any chemical reactions between the MoS and the steel or DLC coating was observed. [ABSTRACT FROM AUTHOR]

Various solid lubricant particles have been experimentally evaluated as possible additives to oils. However, information in terms of a direct comparison of their tribological properties is still missing. In this study, we have compared the tribological properties of seven different solid lubricant micro- and nanoparticles as additives in polyalphaolefin (PAO) oil: MoS nanotubes, MoS platelets (2 and 10 μm), WS nanotubes, WS fullerene-like nanoparticles, graphite platelets (20 μm) and multi-walled carbon nanotubes. The experiments were performed in the boundary lubrication regime under a contact pressure of 1 GPa (Hertz, max) using a ball-on-disc tribotester. In general, the particles significantly decreased the friction and wear compared to the base PAO oil. We found that it was the material of the particles that largely determined their tribological performance. The effect of the size of the particles was much less important, and the morphology (shape) of the particles had little or no influence. We have also investigated the effect of ultrasonication during suspension preparation on particle damage and found that the solid lubricant particles were not notably affected, except the MoS and WS nanotubes, which became somewhat shorter. [ABSTRACT FROM AUTHOR]

ABSTRACT Ionic liquids are low-melting-point salts composed entirely of ions, and many of them are liquids at room temperature. In recent years, studies have indicated that they might be good candidates for lubricants, either in neat or additive form. In this work, a sulfate-based ionic liquid with a pyrollidinium cation was studied as neat lubricant and as additive for glycerol in lubrication of steel-steel contacts. Glycerol was chosen as the base oil because of its high polarity, which allows full miscibility with polar ionic liquids. Tests were performed on an oscillating friction and wear tribometer. The coefficients of friction and wear were measured. The tests were run at room temperature, 50 °C, 100 °C and 150 °C. By using profilometry, optical microscopy, scanning electron microscope and atomic force microscopy (AFM) analyses, it was shown that the ionic liquid plays an important role in the friction and wear reduction, as well as in the smoothening of the worn surface. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Highlights: [•] Wetting properties of different materials with oils and water are investigated. [•] Type of wetting plays an important role in wetting evaluation. [•] Spreading parameter correlates linearly with the surface energy. [•] Spreading parameter is a reliable tool for tailoring the wetting properties. [ABSTRACT FROM AUTHOR]

The use of tap water instead of the conventional hydraulic fluids in power-control hydraulics is one of possible environmentally friendly changes that we can make. In this paper we show, based on dynamic-transient parameters, both the functionality and the usability of water hydraulics in comparison to the more familiar oil hydraulics. A comparison of the dynamic behavior, oriented to pressures, between the conventional mineral hydraulic oil and the relatively new water hydraulics under the same conditions is described here. The tests were made at different flow rates (11, 22 and 33 lpm), different system pressures (70, 110 and 160 bar) and with different loads (first with a mass of 163 kg in the horizontal and vertical positions and second without this mass). Different dynamic characteristics during the pressure-surge effect of water and oil hydraulics were observed. The pressure increases were between 10 and 30 % higher in the water hydraulics than in the oil hydraulics. The frequencies of the pressure oscillations were up to 20 % lower in the water hydraulics than in the oil hydraulics. [ABSTRACT FROM AUTHOR]

Highlights: [•] Neutron-reflectometry was used to study alcohol adsorption on various DLC coatings. [•] Thickness and density of alcohol adsorbed layers were determined. [•] Relatively dense 0.9-nm-thick adsorbed layer was detected on a-C coating. [•] 1.3-nm-thick, but less dense, adsorbed layer was detected on Si-doped a-C:H coating. [•] No adsorbed layers were detected on a-C:H and F-doped a-C:H coating. [Copyright &y& Elsevier]

The application of diamond-like carbon (DLC) coatings on the contacts of mechanical systems is becoming widespread thanks to their excellent tribological properties. Numerous studies of DLC coatings have been reported over the past decade and, as a result, the understanding of their lubrication has improved. The tribological properties of boundary-lubricated contacts are drastically affected by adsorbed layers; however, due to the variety of lubricant additives and coating structures, no general adsorption mechanisms for DLC coatings have been put forward until now. This has, unfortunately, hindered improvements in their lubrication performance. Many of the essential physical properties of the adsorbed layers also remain undefined. In this work, we used neutron reflectometry to determine the thickness and the density of the adsorbed layers of fatty acid molecules on coatings of a-C, a-C:H, a-C:H:F and a-C:H:Si. The results showed that a 0.9-nm-thick layer adsorbed onto the a-C and a-C:H coatings. In contrast, both doped coatings, i.e. the a-C:H:F and a-C:H:Si, showed a poorer adsorption ability towards the fatty acid molecules than the a-C and a-C:H. Continuous adsorption layers were not detected on the a-C:H:F and a-C:H:Si; however, the possibility of adsorption in lower quantities cannot be ruled out. [ABSTRACT FROM AUTHOR]

In any attempt to theoretically calculate the real contact area for 3D engineering surfaces, a criterion is needed to identify the relevant asperity-peaks that carry the load in tribological contacts. In our recent work, we investigated how different, available 2D criteria affect the properties of the theoretically determined asperity-peaks in 2D surfaces. In this work, however, we focused on a 3D surface characterisation. The effect of different asperity-peak identification criteria on the properties of the asperity-peaks (numbers, radii and heights) is studied in the 3D domain. Several different criteria that take into account the number of neighbouring points, the distances between them (lateral resolution) and their heights were evaluated for real measured surfaces with five different surface roughnesses in the broad engineering range of arithmetic surface roughness from S a =0.005µm to S a =0.529µm. From the results it follows that all three chosen asperity-peak identification criteria (5PP-3D, 9PP-3D and 9PP-R-3D) result in reliable asperity-peak properties, and none of them can be favoured based on a theoretical evaluation only. There are, however, important differences between them. The data resolution in the x and y directions has a very important influence on the numbers, radii and heights of the asperity-peaks, and the results suggest that the data's lateral resolution, below 1µm, should be used for the relevant asperity-peak identification. [ABSTRACT FROM AUTHOR]

This work shows the influence of solid-liquid interactions between engineering surfaces (steel and several types of DLC coatings) and lubricating oil (polyalphaolefin, PAO) on the coefficient of friction in the elastohydrodynamic lubrication (EHL) regime. Specifically, it confirms that the spreading parameter, rather than the contact angle, is the relevant parameter to evaluate the wetting behaviour of these surfaces with oils. Both the spreading parameter and the surface energy correlate very well with the friction in the EHL regime and can predict its behaviour. In particular, the polar component of the surface energy was found to correlate almost perfectly with the friction behaviour (a Pearson's linear correlation coefficient of 0.999). By tailoring the wetting and surface energy-achieved by varying the DLC/DLC contacts with different types of DLC coatings-the coefficient of friction in the EHL regime was reduced by more than 30 % compared to steel/steel contacts. Poor wetting of the DLC coatings with a low surface energy is reflected in low values of the spreading parameter, which indicates easier slip of the lubricant over the solid surface due to shear action, and this leads to a lower viscous friction. A 'Slip-inducing interaction model based on surface forces' is presented to explain why oil slip is promoted, particularly at surfaces with a low polar surface energy. The model suggests that a small number of permanent polar interactions, i.e. a larger proportion of intermittent dispersive interactions, results in less adhesive interactions between the predominantly non-polar liquid (oil) and the low polar surface (DLC), which enables easier slip at the solid-liquid interface. [ABSTRACT FROM AUTHOR]